NMR neck coil with passive decoupling

ABSTRACT

A whole volume neck coil for MRI includes an anterior and posterior saddle coil, the latter attached to a base. The anterior coil is held in an open framework, hinged against an extension tower attached to the base. The hinge axis is angled to the medial axis to provide greater entry area for the patient&#39;s head and shoulders. The extension tower also provides adjustment of the anterior coil toward and away from the base by means of a sliding carriage attached to the extension tower. The coils are passively decoupled from the RF excitation field by means of back-to-back diodes which insert a &#34;pole&#34; forming network into the coils. A method of calculating the values of the network components is taught that accounts for significant diode junction capacitance.

This is a division of application Ser. No. 07/601,552, filed Oct. 22,1990 now U.S. Pat. No. 5,166,618.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is magnetic resonance imaging (MRI) and inparticular local coils for use in receiving MRI signals.

2. Background Art

In MRI, a uniform magnetic field B₀ is applied to an imaged object alongthe z axis of a Cartesian coordinate system, the origin of which isapproximately centered within the imaged object. The effect of themagnetic field B₀ is to align the object's nuclear spins along the zaxis.

In response to a radio frequency (RF) excitation signal of the properfrequency, oriented within the x-y plane, the nuclei precess about thez-axis at their Larmor frequencies according to the following equation:

    ω=γB.sub.0

where ω is the Larmor frequency, and γ is the gyromagnetic ratio whichis constant and a property of the particular nuclei.

Water, because of its relative abundance in biological tissue and theproperties of its nuclei, is of principle concern in such imaging. Thevalue of the gyromagnetic ratio γ for water is 4.26 kHz/gauss andtherefore in a 1.5 Tesla polarizing magnetic field B₀ the resonant orLarmor frequency of water is approximately 63.9 MHz.

In a typical imaging sequence, the RF excitation signal is centered atthe Larmor frequency ω and applied to the imaged object at the same timeas a magnetic field gradient G_(z) is applied. The gradient field G_(z)causes only the nuclei in a slice through the object along a x-y planeto have the resonant frequency ω and to be excited into resonance.

After the excitation of the nuclei in this slice, magnetic fieldgradients are applied along the x and y axes. The gradient along the xaxis, G_(x), causes the nuclei to precess at different frequenciesdepending on their position along the x axis, that is, G_(x) spatiallyencodes the precessing nuclei by frequency. The y axis gradient, G_(y),is incremented through a series of values and encodes y position intothe rate of change of phase of the precessing nuclei as a function ofgradient amplitude, a process typically referred to as phase encoding.

A weak nuclear magnetic resonance generated by the precessing nuclei maybe sensed by the RF coil and recorded as an NMR signal. From this NMRsignal, a slice image may be derived according to well knownreconstruction techniques. An overview NMR image reconstruction iscontained in the book "Magnetic Resonance Imaging, Principles andApplications" by D. N. Kean and M. A. Smith.

The quality of the image produced by MRI techniques is dependent, inpart, on the strength of the NMR signal received from the precessingnuclei. For this reason, it is known to use an independent RF receivingcoil placed in close proximity to the region of interest of the imagedobject to improve the strength of this received signal. Such coils aretermed "local coils" or "surface coils". The smaller area of the localcoil permits it to accurately focus on NMR signal from the region ofinterest. Further, such local coils may be of higher quality factor or"Q" than the RF transmitting coil increasing the selectivity of thelocal coil and the relative strength of the acquired NMR signal.

The smaller size of the local coil makes it important that the localcoil be accurately positioned near the region of interest. For "wholevolume" coils, employing two antenna loops to receive the NMR signalfrom a volume defined between the loops, accurate positioning of thecoils is particularly important. For a whole volume neck coil, forexample, the two antenna loops must be placed on opposite sides of theneck and yet generally opposed along a single axis. This may beaccomplished by fitting the coils to the surface of a cylindrical form,the form parting along a plane intersecting the axis of the cylinder toform two halves. These halves may be clamped about the patient duringthe scan.

One problem with this method of mounting the coils is that theseparation of the coils is fixed by the radius of the cylinder. Equallyimportant, the joining of the two halves of the form creates a closedcage that may create a disquieting sense of confinement.

A major technical problem in NMR systems is "decoupling" the local coilfrom the RF excitation signal from the transmit coil during theapplication of the RF excitation signal. Such decoupling reduces thedistortion of the excitation field by the local coil and preventspotential damage to the sensitive circuits connected to the local coilfrom possibly large induced voltages.

Inductive coupling between the excitation field and the local coil mayfocus the deposition of the RF energy on a reduced volume the imagedobject. In the case of the medical imaging of a patient, such focusedenergy may cause burns. Energy coupled to the local coil itself maycause heating of that coil, producing indirect burns to the patient anddamage to the local coil and its circuitry. The problem of distortionand inductive coupling is compounded by the typical high "Q" of thelocal coils.

One method of decoupling the local coil from the RF excitation field isthrough the use of one or more solid state switches positioned along thelocal coil which may be activated either by an external electricalsignal (active decoupling) or by the RF excitation field itself (passivedecoupling). These switches disable or detune the local coil. One suchapproach which shows generally the use of back-to-back diodes forpassively decoupling a local coil is described in U.S. Pat. No.4,725,779, issued Feb. 16, 1988 to Hyde et al., entitled: "NMR LocalCoil with Improved Decoupling" and hereby incorporated by reference. Inthis reference, back-to-back diodes, in the presence of the largeinduced voltages from the transmit coil, short together two adjacentantenna coils having counter rotating currents thus decoupling theantenna coils from the RF excitation field and reducing the inductivecoupling to the local coil. The advantage of passive decoupling is theelimination of the need for additional wires and signals to control thedecoupling device and hence the simplification of the coil.

SUMMARY OF THE INVENTION

The present invention provides an improved local coil for receiving NMRsignals from the vicinity of a patient's neck. A base supports thesupine patient's neck and head along a medial axis and a posteriorsaddle coil fits beneath the patient's head and neck. The posterior coilhas a first and second arcuate end segment joined by a first and secondloop. The arcuate end segments are attached to the base and the loopsextend upward from the base so as to receive the patient's neck. Asimilar anterior saddle coil receives the patient's neck in substantialopposition to the first saddle coil when the anterior saddle coil is inthe closed position.

The anterior saddle coil may open on a hinge which has a hinge axiscrossing the medial axis and which is attached to the base by means ofan extension tower extending upward from the base. The extension towermay include a sliding carriage for adjusting the hinge and hence theanterior coil toward and away from the base and posterior coil. Thesliding carriage may be moved by means of a rack and pinion

It is thus one object of the invention to provide a coil that reducesthe patient's sense of being "closed in". The open construction of theanterior coil gives the coil a lightweight appearance as does theextension tower which provides the sole support for the anterior coil.The single point hinging of the anterior coil on the extension toweralso improves the patient's sense of being unrestrained.

It is another object of the invention to proved an improved method ofpositioning the anterior coil and posterior coil with respect to thepatient. The tipped axis of the hinge increases the entry area of thecoil when the anterior coil is in the open position to conform to theswept volume of the patient's head and neck when the patient lies backagainst the base and posterior coil. The sliding carriage on theextension tower permits the height of anterior coil to be adjustedclosely to the neck for improved sensitivity but preserves the relativeposition of the anterior and posterior coils along a constant coil axis.A rack and pinion allows this adjustment to be performed quickly andeasily with one hand.

An improved passive decoupling method for such local coils is alsotaught in which the coil is effectively broken by the insertion of ahigh impedance network into the coil loop ("pole insertion")Specifically, a capacitor is connected across the terminals of theantenna coil for tuning the antenna coil to the resonant frequency. Anon-linear conductor with a conducting and non-conducting state andhaving an intrinsic capacitance in the non-conducting state is placed inseries with an inductor and the series combination is connected acrossthe capacitor so as to form, in combination, a high impedance network.The value of the capacitor, the inductor and the intrinsic capacitanceof the non-linear conducting element are selected to maximize theinpedance across the network, at the resonant frequency, and thecapacitance and intrinsic inductance of the antenna loop are chosen sothat the magnitude of their impedances are substantially equal at theresonant frequency.

It is thus one object of the invention to provide an improved method ofdecoupling a local coil by pole insertion. It has been discovered thatthe capacitance of the non-linear element, typically back-to-backdiodes, has a significant effect on the selection of the proper valuesfor the other network components. In particular the intrinsiccapacitance of the network must greater than four times the intrinsiccapacitance for optimum decoupling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the local coil of the present inventionshowing the posterior and anterior paddle in the open position;

FIG. 2 is an elevation in cross-section along a midsagittal planeshowing the positioning of the local coil of FIG. 1 on a patient;

FIG. 3 is a plan view of the local coil of FIG. 1 showing the anteriorand posterior coils in the closed position;

FIG. 4 is a perspective view of the anterior and posterior antenna loopsin the closed position with the supporting structure removed forclarity;

FIG. 5 is a cross-sectional view of the local coil along line 5--5 ofFIG. 3;

FIG. 6 is a cross sectional view of the extension column supporting theanterior coil taken along line 6--6 of FIG. 3;

FIG. 7 is a schematic or the anterior and posterior coils and theirassociated circuitry; and

FIGS. 8(a) and 8(b) are simplified versions of the schematic of FIG. 7during the RF transmit mode and the NMR receive mode respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the neck coil 10 of the present invention includesopposed anterior and posterior coils 12 and 14. The posterior coil 14 issupported by a generally horizontal, planar base 16 whereas the anteriorcoil 12 is held away from the base 16 by an extension tower 18projecting perpendicularly upward from the horizontal surface of thebase 16.

Referring also to FIG. 2, when the local coil 10 is in use, thepatient's head rests back against the upper surface of the base 16 withthe patient's frontal plane generally parallel to the surface of thebase 16. Left and right medially extending wedges 20 rise from the uppersurface of the base 16. The wedges are symmetrically opposed about themedial axis 22 to support and position a trough shaped cushion 24 thatcradles either side of the patient's neck and head when the patient ispositioned in the coil 10. Held within the trough is a transversehemi-cylindrical foam pad which supports the back of the patient's neckand tips the patient's head to face substantially upward on the base 16.Flat foam cushions 28 are positioned against the base 16 above and belowthe hemi-cylindrical cushion 26 along the medial axis 22, to support theback of the patient's head and shoulders.

The anterior antenna coil 12 includes an arcuate nose arch 30 and chestarch 32 joined to each other at the arch ends by substantially straightleft and right sidebar 34 and 36 which form a saddle shaped guide. Theleft sidebar 34 is attached to and may hinge about the extension tower18, as will be described, so as to move the anterior antenna coil 12into an open or closed position. In the closed position, the left andright side bars 34 and 36 are parallel to the upper surface of the base16 and positioned above the left and right wedges 20. In the closedposition, the nose arch 30 extends downward around the patient's face ina transverse plane, and the chest arch 32 extends downward around thepatient's lower neck in a plane angled between the transverse andfrontal planes.

Referring to FIG. 3, the left sidebar 36 of the anterior antenna coil 12includes an angled support 38 which forms one half of a hinge 40 havinga hinge axis 42 parallel to the base 16 but tipped 30° with respect tothe medial axis 22 so as to cross over the medial axis 22 superior tothe patient's neck. This tipping of the hinge axis 42 retracts the chestarch 32 away from the point of patient entry when the anterior coil 12is moved to the open position, thus improving the access for the patientwho normally lays back against the base 16 and whose head enters thecoil at a relatively steep angle. The above structural components arefabricated from a non-magnetic, non-conductive, polymeric material toreduce their interaction with the magnetic and electrical fields of theMRI equipment.

Referring to FIG. 4, the anterior and posterior antenna coils 12 and 14respectively carry an anterior and posterior antenna loop 44 and 46formed of a length of copper tubing. In the closed position, theanterior and posterior antenna loops 44 and 46 form two loops of aHelmholtz solenoid having a solenoid axis 47 normal to the surface ofthe base 16 for detecting a vertically oriented RF field. The anteriorand posterior antenna loops 44 and 46 are connected together to form aresonant structure by means of cable 48 as will be described furtherbelow.

Referring to FIGS. 1 and 4, the anterior antenna loop 44 is supported bythe nose arch 30, the side bars 34 and 36, and the chest arch 32 andconforms to the frame so created. Cable 48 attaches to the anteriorantenna loop 44 at the left sidebar 34 and exits through a hole drilledin the angle bracket 38 (shown in FIG. 3). A strain relief 50 guides thecable 48 in a loop to provide room for flexure of the cable 48, withopening and closing of the anterior antenna coil 12 and with verticalmovement of the hinge 40, as will be described below. The cable 48 isthen received by a second strain relief 52 in the vertical wall of theextension tower 18.

Referring to FIGS. 3 and 6, the extension tower 18 incorporates asliding carriage 56 fitting within a vertical slot 58 (visible in FIG.2) on a face toward the anterior antenna coil 12. The sliding carriage56 forms the second half of the hinge 40. The sliding carriage 56 issupported against rotation by the walls of the slot 58 and may beadjusted toward and away from the base 16 along the slot 58. Thus theheight of the anterior antenna coil 12 above the patient's head may beadjusted, when the anterior antenna coil 12 is in the closed position.The slot 58 extends parallel to the solenoid axis 47 to preserve theorientation of the anterior and posterior antenna loops 44 and 46 alongthat axis 47 with motion of the carriage 56.

The sliding carriage 56 is attached to a cylindrical rack 59 containedin the extension tower 18 which is engaged by a pinion gear 60 attachedto a knob 62 which protrudes from a face of the extension tower 18opposite to that through which the sliding carriage 56 extends. A rubberO-ring 63 fits on the cylindrical end of the rack 59 and rubs againstthe channel through which the rack 59 slides to prevent slipping of therack 59 after it has been positioned by the pinion 60 and knob 62. Theheight of the anterior coil 12 may be adjusted with one hand by turningthe knob 62 appropriately.

The cable 48 from the anterior antenna coil 12 passes through the strainrelief 52 into a second channel in the extension tower 18. The extensiontower 18 is attached to the base 16 and the cable 48 passes through ahole in the base 16 at the point of attachment to connect to theposterior antenna loop 46.

Referring to FIGS. 4 and 5, the posterior antenna loop 46 includes twoarcuate loops 64 which rise on either side of the patient's neck whenthe patient is in position on the coil 10. These loops 64 fit intochannels in the left and right wedges 20. The left and right loops 64are connected by upper and lower end segments 66 substantially parallelto the plane of the base 16 and contained beneath the base 16 in aprotective housing 68. A pickup loop 70 nests within the upper suchsegment 66 to provide inductive coupling to the posterior antenna loop46 as will be described.

The base 16 has on its lower surface a number of downwardly extendingarc shaped ribs 72 that fit against the concave upper surface of the MRItable 74 (shown in FIG. 1) to stiffen the base 16 and to provideadditional support for the base 16 against the table 74. Also shown inFIG. 1, the base 16 includes cut out hand grips 76 which permit it to bereadily removed from the table 74.

Referring to FIGS. 4 and 7, the posterior and anterior antenna loops 44and 46, as mentioned, are constructed of saddle shaped lengths of coppertubing. The pickup loop 70 is formed from a half circle of copper wireplaced within the circumference of the posterior antenna loop 46 so asto inductively couple to the posterior antenna loop 46. The pickup loop70 is opened at one point along its circumference near the extensiontower 18 to form two terminals 72 and 74 which are attached by means ofcable 76 to the NMR equipment (not shown). As has been described, cable48 is connected between the anterior and posterior antenna loops 44 and46. Both cable 76 and 48 are standard 50 Ω co-axial cable.

The anterior antenna loop 44 is electrically identical to the posteriorantenna loop 46, except for the absence of a pickup loop 70 and itsassociated circuitry, and for this reason, only the circuitry of theposterior antenna loop 46 will be described in detail. The absence ofthe pickup loop 70 from the anterior antenna loop 44 allows the anteriorantenna coil 12 to have an open construction to reduce patientapprehension or claustrophobia as has been discussed.

The posterior antenna loop 46 is cut at four points spaced approximatelyequal distances along the loop. The cuts are bridged by capacitors 78through 84. These cut point will be termed interfaces and the capacitors78 through 84 across the cut points will be termed interface capacitors.

The interface at capacitor 84 along the left lateral edges of the base16 near the extension tower 18 receives the cable 48 joining theanterior and posterior antenna loops 44 and 46. The shield and centerconductor of the cable 48 are connected on either side of the interfacecapacitor 84 and provide a DC connection to a corresponding interfacecapacitance on the anterior antenna loop 44 such that the currentthrough the anterior and posterior coils 44 and 46 flows in the samedirection at resonance. A tuning capacitor 86, positioned in theextension tower 18 midway along the cable 48 connects the shield andinner conductor of cable 48 and serves to tune both the anterior andposterior antenna loops 44 and 46, in tandem, to the resonant frequency.Differences between the relative tunings of the anterior and posteriorantenna loops 44 and 46 are removed by a trimmer capacitor connectedacross one of the interface capacitors of the anterior antenna loop aswill be understood to those of ordinary skill in the art.

Short leads connect to the posterior antenna loop 46 on each side of theinterface associated with interface capacitor 80, opposed to theinterface capacitor 84, at either end of series connected back-to-backdiodes 88 and inductor 90 which together with capacitor 80 comprise thepassive decoupling network 92.

As is understood in the art, the back-to-back diodes 88, are constructedof two diodes having the anode of the first connected to the cathode ofthe second and the cathode of the first connected to the anode of thesecond to form a non-linear conductor which has a very high resistancefor voltages below a forward bias voltage (approximately 0.7 volts forsilicon diodes) and a low resistance for voltages above this threshold.When the voltage across the interface capacitor 80 in the posteriorantenna loop 46 is below the forward bias threshold of the back-to-backdiodes, as will be the case when NMR signal is being detected, theinductor 90 will be connected in series with the posterior antenna loop46.

As mentioned, the purpose of the passive decoupling network is todecouple the posterior antenna coil 46 from the high strength RFexcitation field but to couple the posterior antenna coil 46 to the lowstrength NMR signal both at the same resonant frequency. This decouplingis accomplished by the above described switching of inductor 90 into theposterior antenna loop 46 in response to high loop currents caused bythe RF excitation field. The inductor 90 together with capacitor 80 forma "pole" or band reject network tuned to the resonant frequency andeffectively blocks current flow at that resonant frequency and decouplesthe posterior antenna loop 46.

As described, the switching of the inductor 90 into and out of theposterior antenna loop 46 is accomplished by the back-to-back diodeswhich conduct only in response to the higher voltages induced by the RFexcitation field and are essentially non-conductive at the lowervoltages induced by the NMR signal. Each of these states: 1) conductingduring RF excitation, and 2) not conducting during NMR signal receptionchanges the effective composition of network 92 and thus thecharacteristics of the posterior antenna loop circuit as is shown insimplified schematics of FIGS. 8(a) and (b).

Referring to FIGS. 7 and 8(a), during the reception of the NMR signal,the back-to-back diodes 88 are non-conducting. Rather than completelydisconnecting the pole-forming inductor, however, as might beanticipated by a simple model of such diodes 88, it has been determinedthat they exhibit a significant junction capacitance represented byC_(j). Hence the passive network 92 of FIG. 8(a) consists of aneffective capacitance of value C_(j) in series with the inductor 90 ofvalue L₁, with that series combination connected across capacitor 80having value C₁.

The entire network 92 is connected across the rest of posterior antennacoil 46 having a net inductance value of L. While each antenna loop 44and 46 also includes a number of interface capacitors and the impedanceof the other antenna loop 46 or 44 coupled inductively and through cable48, the impedance as seen at the interface of capacitor 80 will beinductive at the resonant frequency as a result of the tuning of thelocal coil 10 to that resonant frequency. Specifically, in the case ofthe posterior antenna loop 46, this tuning results in approximateequivalence between the impedances of all four interface capacitances:78, 80, 82, 84 and the distributed inductance of the posterior antennaloop 46. Hence, the reactance of the posterior antenna loop 46 seenacross any one of its interfaces, includes only three of the interfacecapacitors, and will appear inductive.

Referring to FIGS. 7 and 8(b), during the transmission of the RFexcitation signal, the back-to-back diodes 88 conduct and hence shuntthe junction capacitance C_(j) reducing the network 92 to a simpleparallel resonant circuit comprised of capacitor 80 (C₁) and inductor 90(L₁). Ideally, the values of these two components are selected to createa filter having a pole at the resonant frequency.

Recognizing the significance of the capacitance C_(j) of theback-to-back diodes 88, the relative values of C_(j), C₁, L₁ and L maybe determined. The following analysis is identical for the anterior andposterior antenna loops.

As stated above, during the receiving of the NMR signal, theback-to-back diodes 88 are in their high impedance state and hence thecircuit associated with each coil may be simplified to the diagram asshown in FIG. 8(a).

The impedance of C_(j), C₁, and L₁ is capacitive in character at thefrequency of the NMR signal, (i.e. near the resonant frequency). In thiscase, the entire network 92 will have a purely capacitive impedance withmagnitude defined as X_(C). Also, for the purpose of this analysis, theresistance of the various elements of the network 92 and posteriorantenna loop 46 are neglected. It will be understood to those ofordinary skill in the art that such small resistances of high qualitycomponents will have negligible effect on the pole frequency of thenetwork 92 or the resonant frequency of the posterior antenna coil 46which together define the values of C_(j), C₁ and L₁.

Given the above assumptions, the impedance of the network 92 when thelocal coil 10 is receiving and the back-to-back diodes 88 arenon-conducting is: ##EQU1##

where -X_(C) is the impedance of the network 92 and X_(C).sbsb.1,X_(C).sbsb.j, and X_(L).sbsb.1 are the impedances of diodes 88, thecapacitor 80 and inductor 90 respectively, and where j=√-1 perconvention.

As is understood in the art, the posterior antenna loop 46 is tuned tothe resonant frequency so as to be maximally sensitive to the NMRsignal. It is understood that this tuning requires that |-jX_(C)|=|jX_(L) | where X_(L) is the inductance of the posterior antenna loop46.

As mentioned above, when the posterior antenna loop 46 is in thepresence of an exciting RF field, the back-to-back diodes 88 conduct,effectively shorting the capacitance C_(j) and producing the equivalentcircuit of FIG. 8(b). For optimal decoupling by pole insertion, thevalues L₁ and C₁ must be selected to form a parallel resonant circuit atthe excitation or resonant frequency to block current flow through theposterior antenna loop 46. Hence:

    |X.sub.C.sbsb.1 |=|X.sub.L.sbsb.1 |(2)

Substituting equation (2) into equation (1) and simplifying yields##EQU2## then by definition ##EQU3## and therefore: ##EQU4##

Of course capacitance values C, C₁, and C_(j) must be positive and realand hence equation (7) demands

    C>4C.sub.j                                                 (8)

For a given junction capacitance C_(j) of the back-to-back diodes 88,this condition may be met by changing C₁ and correspondingly changingthe value of the inductance of the posterior antenna coil 46 as isappropriate. Increasing the inductance of the posterior antenna coil 46,however, is realized by a change in coil geometry or size and henceeither reduces the Q of the local coil 10, or degrades the fillingfactor or sensitivity of the coil to signal from the specified patientanatomy. Therefore, it is preferable to find diodes 88 with the smallestvalues of C_(j) possible and with the appropriate switching speeds.

Determination of the values of C₁, L₁, and L, as derived above, providessuperior decoupling and tuning for the neck coil 10.

It should be noted that in general, the value of capacitor C₁ is fixedby the inductance of the posterior antenna loop, that inductive valuebeing a function of the geometry of the coil. Further, the value C_(j)is determined by the manufacture of the diodes 88 and fixed, within arange, for a given diode type. Although the inductance L₁ is tunable, itcannot compensate for incorrect values of C₁ or C_(j) as is apparentfrom equation 7. Therefore, as a practical matter, it is necessary tocalculate the proper values of the network 92 components prior toconstruction of the coil 10--they cannot be "tuned" once in place. Forthis reason, it is unlikely that the correct values would be arrived ataccidently or by trial and error.

The received NMR signal is transmitted to the MRI apparatus by cable 76which is connected to the pickup loop 70. A parallel capacitance 94,connected across cable 76 through a series capacitance 98, between onewire of the cable and the parallel capacitance 94, provides impedancematching of the pickup loop 70 to the 50 Ω cable. A second pair ofback-to-back diodes 96 shunt the capacitance 94 as well as short thepick-up loop 70. Capacitor 98 also prevents DC bias, which may bepresent on cable 76, from inadvertently switching diodes 96. The diodes96 serve to detune the pickup coil so as to reduce direct pickup of theRF excitation signal by the pickup coil despite decoupling of theanterior and posterior coils 12 and 14.

The above description has been that of a preferred embodiment of thepresent invention. It will occur to those who practice the art that manymodifications may be made without departing from the spirit and scope ofthe invention. For example, the passive decoupling network described maybe applied to other types of local coils which need not be whole volumecoils. In order to apprise the public of the various embodiments thatmay fall within the scope of the invention, the following claims aremade.

We claim:
 1. A local coil for use in receiving NMR signals from thevicinity of a patient's neck, the coil comprising:a base for supportingthe patient's neck and head along a medial axis; a first saddle coilmeans having a first and second arcuate end segments joined by a firstand second loop, the arcuate end segments attached to the base and theloops extending outward from the base so as to receive the patient'sneck therebetween; a hinge having a first and second side and includinga hinge axis crossing the medial axis, the first side affixed to anextension tower attached to and extending outward from the base; and asecond saddle coil means having a first and second arcuate end segmentjoined by a first and second loop, the first loop attached to the secondside of the hinge so as swing between an open and closed position andwherein the first and second loop of the second coil extend inwardtoward the base to receive the patient's neck in substantial oppositionto the first saddle coil when the second saddle coil is in the closedposition.
 2. The local coil of claim 1 wherein the extension towerincludes a carriage for sliding inward and outward along the extensiontower and the second side of the hinge is attached to the carriage. 3.The local coil of claim 2 wherein the position of the carriage isadjusted by means of a rack and pinion gear.